Temperature control system for a diffusion furnace



Dec. 13, 1966 B. J. SPERANSKY ET AL TEMPERATURE CONTROL SYSTEM FOR A DIFFUSION FURNACE Filed Oct. 18, 1963 vi IQ." J v S:

Attorneys United States Patent TEMPERATURE CONTROL SYSTEM FOR A DIFFUSION FURNACE Boris J. Speransky, Menlo Park, and Cecil A. Lasch, Jr.,

Redwood City, Calif., assignors to Electroglas, Incorporated, Redwood City, Calif., a corporation of California Filed Oct. 18, 1963, Ser. No. 317,268 12 Claims. (Cl. 219-503) is, therefore, a need for a new and improved diffusion furnace.

In general, it is an object of the present invention to provide a diffusion furnace having an improved tempera? ture control.

Another object of the invention is to provide a diffusion furnace of the above character in which a master'and slave system is utilized for controlling multiple zones in a diffusion furnace.

Another object of the invention is to provide a diffusion furnace of the above character in which it is unnecessary to precisely position the thermocouples utilized for temperature control.

Another object of the invention is to provide a diffusion furnace of'the above character in which the'controls for each zone need not be'reset for each temperature change made in the diffusion furnace.

Another object of the invention is to'provide a diffusion furnace of the above character in which longer ffat zones can be provided.

Another object of the invention is to provide a diffusion furnace of the above character in which errors in positioning and variations in outputs of the thermocouples utilized can be readily compensated for.

Another object of the invention is to provide a diffusion 3,291,969 Patented Dec. 13, 1966 ance to the galvanometer and which also makes it possible to adjust through a substantial range the signal supplied to the control means for the master zone. A galvanometer is also provided for each of the other zones. The signal to the galvanometer is obtained by placing one thermocouple in the -master zone and thenplacing another thermocouple in the other zone to be controlled. Means is provided for connecting the output of the galvanomteer to the zone to control the temperature of the zone so that it is slaved to the master zone. This means also includes means which presents a constant output impedance to the galvanometer and which makes it possible to adjust through a substantial range the signal supplied to the contr-olmeans for the associated zone.

As shown in the drawings, our diffusion furnace 10 consists Of heating means in the form of a single coil 11 which extends through a plurality'of zones, namely, zones A, B and C for one individual furnace. Alternatively, a separate coil can be used for each zone if desired. This coil 11 is supplied with power through separate temperat-urecontrol means for each zone from a suitable source such as single phase A.-C. of a suitable voltage such as 208 volts as indicated as being connected to the lines L1 and L2. Normally, a plurality of such individual furnaces are provided in one location as, for example, three furnaces, so that a three-phase supply can be utilized.

' Each of the zones or sections of the coil 11 is connected to a secondary 12 of one of the voltage reducing transformers TR1, TR2 and TR3. The primary 13 of each of the transformers TR1, TR2 and TR3 has one side furnace of the above character in which it is unnecessary to readjust the galvanometers for each temperature change which is made for the diffusion furnace.

Additional objects and features of the invention will appear from the following description in which the preferred embodiment is set forth in conjunction with the accompanying drawing. 9

The drawing is a circuit diagram with certain parts schematically illustrated of a diffusion furnace incorporating our invention.

In general, our'diffusion furnace consists of a plurality of zones With temperature controlling means associated with each zone. The temperature controlling means for each zone includes control means which connects the source of power to each zone. A set point controller is used for one of the zones which can be called a master zone. A thermocouple is provided in the master zone and is connected to the set point controller and supplies a signal to a galvanometer which produces an output signal proportional to the deviation between the measured temperature in the master zone and the set point. Means is connected to the output of the galvanometer and to the control means for the master zone for controlling the application of power to the master zone. This means includes means which presents a constant output imped.

connected to a line 14 which is connected to a circuit breaker CB. As shown, the circuit breaker CB is adapted to connect line 14 to line L1v when it is closed. The other side of each of the'primary windings 13 is connected to one side of the output winding of the associated saturable core'reactor in which the sat-urable core reactors are identified as SCRI, 'SCRZ and SCR3, respectively, for the three zones. The other side of the output winding of the saturable core reactor is connected to a line 16 which is also connected tofthe circuit breaker CB and is adapted to be connected to the line L2. i 7

As is well known to those skilled in the art, there will be some voltage 'drop across the saturable core reactor, and for that reason, the voltage supplied to the primary winding 13 of the transformers TR1, TR2 and TR3 will be approximately 193-195 volts, assuming that 208 volts is supplied to the saturable core reactor. With such a voltage, the secondary winding 12 of the transformers TR1, TR2 and TR3 will deliver a suitable low voltage such as approximately 30 volts to sections or zones A, B and C of the coil 11. I I

A voltmeter V1 is connected across the input winding of each of the transformers TR1, TR2 and TR3 for measuring the input voltage to. the transformers. Arne meters A-1, A2 and A3 are provided in the line from the saturable 'core reactors to the input or primary windings of the transformers TR1, TR2 and TR3 to measure the current flow into the transformers. I

The input windings of the saturable core reactors SCRl, SCRZ and SCR3 are each connected to a magnetic amplifier. These magnetic amplifiers are identified as MAI, MA2 and MA3, respectively. The magnetic amplifiers can be of any suitable type such as Model No. 6191 supplied by the Minneapolis-Honeywell Regulator Co., Industrial Products Group, Philadelphia, Pa. As is well known to those skilled in the art, the magnetic amplifier supplies a D.-C. current to the D.-C. winding of the saturable core reactor to control the current flow through the saturable core reactor and thereby to control the current flow in each of the zones A, B and C of the River, Mass.

heater winding 11. Such magnetic amplifiers act as power amplifiers and serve to boost low level input sign-alsfrom a primary controller to a high level output to control a saturable core reactor. Such a device supplies a continuous stepless and contactless electrical control.

The input to each of the magnetic amplifiers is connected to a resistance network. As shown in the drawing, these resistance networks are identified as 21, 22 and 23 for the three magnetic amplifiers. A primary controller or null galvanometer is connected to each of the resistance networks 21, 22 and 23 and is provided for controlling the magnetic amplifier. These galvanometers are identified as G1, G2 and G2. The null galvanometer can be of any suitable type such as the Model 105R2l2 Pyr-O-Volt current proportioning controller manufactured and sold by the Minneapolis-Honeywell Regulator Co. of Fall River, Mass. As is well known to those skilled in the art, such a proportioning controller provides a continuous indication of the value of the input variable and produces an output current proportional to the deviation of the indicating point within the proportional band from the desired control or set point.

The input of the null galvanometer G1 is connected to thermocouples T1 and T2 by conductors 26 and 27. The null galvanometer G3 is connected to thermocouples T3 and T4 by conductors 28 and 29. As can be seen from the drawing, thermocouples T1 and T4 are positioned in the zones A and C, respectively, whereas the thermocouples T2 and T3 are positioned in zone B of the winding 11. The input of the null galvanometer G2 is connected to a set point controller 31 through conductors 32 and 33. The set point controller 31 is connected to a master thermocouple T5 by conductors 34 and 36. Any suitable set point controller can be utilized such as the digital set point unit manufactured and sold by the Minneapolis-Honeywell Regulator Co. of Fall As is well known to those skilled in the art, such a digital set point unit is an adjustable potentiometer circuit wherein a manually adjusted set point voltage serving as a constant reference voltage source is presented in opposition to the varying input voltage supplied by the master thermocouple T5 positioned in zone B. The difference or deviation between these two voltages is the output of the digital set point unit and is utilized to deflect the galvanometer G2. The galvanometers G1, G2 and G3 are provided with a center-zero scale so that plus or minus deviation from the set point can be read directly in scale divisions.

The resistor networks 21, 22 and 23 are identical as shown in the drawings. Thus, each resistor network is comprised of an Lapad consisting of potentiometers P1 and P2 and precision adjusting means consisting of potentiometer P3. Potentiometers P1 and P2 are provided with wipers 38 and 39 which are mounted in such a manner that when the wipers are moved in unison, one is moved in one direction and the other is moved in the opposite direction with respect to the windingsof the otentiometers. One end of the winding of potentiometer P1 is connected to the output of the galvanometer, and similarly one end of the winding of potentiometer P2 is connected to the output of the galvanometer and the input of the magnetic amplifier. The other ends of the windings are free. One end of the winding of potentiometer P3 is connected to the wipers 38 and 39 of potentiometers P1 and P2. The other end of the winding is free. The wiper 41 for the potentiometer is connected to the input of the magnetic amplifier. A connection is made from the free end of the potentiometer winding to the wiper 41 so that no open circuit can occur in case the wiper 41 loses contact with the winding.

Operation and use of the diffusion furnace shown in the drawing may now be briefly described as follows. Let it be assumed that the furnace is cold and that it is desired to start up the furnace. In order to control the amount of current which flows in the coil 11 when it is take place.

cold, the control means for each zone is operated to greatly reduce the possible current flow.

Thus, before supplying power to the diffusion furnace, the sliders 38 and 39 of the L-pad of each temperature controlling means are positioned so that all of the impedance of the potentiometer P1 is placed in the control circuit to the magnetic amplifier to greatly reduce, or reduce to zero, the signal from the galvanometer to the magnetic amplifier. Such a reduced signal, when applied to the saturable core reactor, will not cause saturation of the saturable core reactor. This means that the conductivity of the saturable core reactor will be very low, and for that reason, a relatively low current is supplied to the associated transformers and to the associated section or zone.

Since the master thermocouple T5 is connected to the set point controller 31, and since when the furnace is cold, the current flow through the master thermocouple T5 will be much greater than normal to give an indication that the temperature is far below the set point given by the set point controller 31. Therefore, a large signal will be supplied to the null galvanometer G2 and nor-' mally, the null galvanometer G2 will provide an output current proportional to the deviation from the desired control or set point. In order to prevent such a large signal from being supplied to the magnetic amplifier, the L-pad resistive network 22 is adjusted to place the maximum impedance of the potentiometer P1 in the circuit to the magnetic amplifier to thereby reduce the signal from the controller G2 to a safe value.

The L-pads of the resistor networks 21 and 23 are adjusted in a similar manner so that only a very small signal will be supplied to the magnetic amplifier from the controllers G1 and G3. I

Because these small signals'are supplied to the magnetic amplifiers, only a small signal will be supplied to the saturable core reactors and, therefore, saturation does not This greatly limits the conductivity of the saturable core reactor so that relatively low currents will be supplied to the transformers TRl, TR2 and TR3 and, therefore, relatively low currents will be supplied to the zones or sections A, B and C of the heating coil 11 of the diffusion furnace.

As the diffusion furnace begins to heat up, the L-pads are manually adjusted to gradually decrease the impedance provided by the L-pads to the signals from the galvanometers or controllers to thereby gradually increase the signal to the magnetic amplifiers and to the saturable core reactors to gradually increase the current flowing in the sections A, B and C of the heating coil 11. This procedure is continued over a considerable period of time as, for example, two hours, until the entire heating coil arrives at the desired temperature as determined by the set point controller 31.

From the foregoing, it can be seen by the use of an L-pad between each of the controllers and the magnetic amplifiers, it is possible to provide zero to regulation of the signal from the controller to the magnetic amplifier. Thus, it can be said that the side of the L-pad facing the magnetic amplifier serves as a regulated impedance side, whereas the other side which faces the controller is a constant impedance side. This constant impedance presented to the output of the galvanometer or controller is provided because the sliders or wipers 38 and 39 move in common and that as the impedance is decreased in one of the potentiometers P1 and P2, it is increased in the other potentiometer. Thus, regardless of the position of the sliders 38 and 39, a constant impedance is presented to the controller so that the setting of the controller or galvanometer is not disturbed.

With the arrangement shown, it can be seen that the central zone B is the master zone and that the thermocouple TS is the master thermocouple which senses the temperature of zone B in the diffusion furnace and that current is supplied through the transformer TR2 until the thermocouple T5 reaches the temperature which is equal to the temperature represented by the set point controller 31.

As the temperature zone B rises, the temperature of the thermocouples T2 and T3 will also rise to provide a differential between the thermocouples T1 and T2, and the thermocouples T3 and T4. Because the polarity arrangement of the thermocouple T1 is opposite the polarity arrangement for the thermocouple T2 as shown on the drawings, and similarly the polarity arrangement for the thermocouple T4 is the opposite of that for the thermocouple T3, the thermocouple having the highest temperature will take over and will cause current to flow into the controller and cause a signal to be supplied to the magnetic amplifier to cause additional current to flow in that section or portion of the heating coil of the diffusion furnace. This will continue until that section of the furnace is brought up to the same temperature as section B.

From this arrangement, it can be seen that zones A and C are slaved to zone B. Zone B temperature is determined by the constant voltage reference source provided in the set point controller 31, and the temperature of zones A and C is determined from zone B.

The potentiometer P3 is provided for each section of the diffusion furnace to make it possible to obtain a high uniformity of temperature within the diffusion furnace and to thereby provide a relatively long flat zone within the furnace so that there will be no temperature gradient across a boat during a diffusion process carried on in the furnace. The potentiometer P3 is provided to make a fine adjustment in the temperature of the associated zone which, for some reason or other, may lag or advance in relation to the other zones in the furnace as, for example, a fraction of a degree or up toapproximately 3 C. The potentiometer P3, in effect, serves as a precision variable resistance which isprovided at the output of the controller beyond the L-pad and at the input side of the magnetic amplifier. The purpose of this precision variable resistor P3 is to absorb some of the signal energy supplied by the controller. This results in a decrease of the output of the magnetic amplifier and thereby decreases the A.-C. power to the corresponding zone of the furnace to thereby yield a corresponding decrease in temperature variable within limits of a fraction of a degree up to a designed maximum value which, for example, may be as much as 3 C.

In order to be able to control the rate of rise of the temperature of the zone controlled by the precision potentiometer P3 as well as the rate of thedrop, it is desirable to provide the potentiometer P3 with a suitable resistance as, for example, 200 ohms, so that it is possible to normally have its wiper 41 positioned in the center to utilize approximately one-half of its vallue. In this way, it is possible to increase or decrease temperature at will merely by rotating the slider 41 from its central position, either clockwise for decreasing the resistance or counterclockwise for increasing the resistance with the arrangement shown in the drawings. With such an arrangement, the temperature in the associated zone of the furnace will always increase with a decrease in the introduced value of the resistance by the potentiometer P3 and decrease with an increase of the value of the resistance introduced by potentiometer P3.

The use of such a precision potentiometer makes it possible to compensate for many factors as, for example, thermocouple aging, minor inaccuracies in the positioning of the thermocouple in the zone, etc.

The connection 42 from the free end of the win-ding of the potentiometer P3 to the center tap of the potentiometer is provided in the event there is a loss of contact between the slider 41 and the potentiometer P3. The diffusion furnace will not be shut down and destroy or seriously affect the processing operation which is being 6 carried on by the diffusion furnace because of a drastic drop in temperature in the diffusion furnace. With such a connection 42, the signal from the controller will not be interrupted because the connection 42 will provide a continuous circuit from the potentiometer P3 to the magnetic amplifier. Thus, if there is a loss of contact between the slider 41 and the potentiometer, a very slight drop in temperature will occur within the zone even assuming that the potentiometer P3- was in minimum position or at zero resistance setting. However, this can be easily compensated for by bringing the other potentiometer P3 for the other zones up to a maximum resistance and then raising the set point controller to compensate for the increased resistance placed in the circuit between the controllers or galvanometers and the magnetic amplifiers.

From the foregoing, it can be seen that we have provided a new and improved diffusion furnace in which proportionality between the primary signal, that is, the thermocouple output volt-age and the output signal from the controller or galvanometer is retained although the signal supplied to the magnetic amplifier can be regulated from zero to without affecting the proportionality because of the constantly matched impedance on the output side of the controller. The use of the fine potentiometer with each of the L-pads also makes it possible to make slight changes in temperature in each of the zones to provide a change of temperature which is not self-recovering in spite of the tendency of the master and slave system utilized in the diffusion furnace to rebalance the supply of energy. This is because of the loss of energy in the precision resistance. Any increase of power supplied by the corresponding controller is constantly cut down at the preset rate which is adjustable practically steplessly in the desired limits from zero to any desired value. The use of such a precision potentiometer in each of the temperature controlling meansfor each of the zones makes possible very fast response. It also provides flexibility and simplicity of control at low cost.

-The use of an L-pad in each of the temperature controlling means for each of the zones is advantageous in that it increases the life of the controllers while still retaining a fast response. A negligible. amount of heat is released by the L-pad because the L-pad regulates a low power signal. The L-pa d arrangement is simple and easy to operate and provides low cost means for controlling each zone of the diffusion furnace.

Although we have described our diffusion furnace with temperature controlling means utilizing magnetic amplifiers and saturable core reactors, it is readily apparent that, if desired, in place of these components, silicon controlled rectifiers can be utilized. In such a case, the output fromthe resistor networks consisting of the L-pads and the precision poten-tiometers would be supplied to the silicon controlled rectifiers for controlling the same. The mode of operation of the master and slave system and the resistive networks connected to the output of the controllers would be substantially identical as hereinbefore described.

We claim:

1. In a diffusion furnace, means forming at least one zone, heating means disposed in the zone, a source of power, a transformer connected to the heating means, a saturable core reactor connected to the source of power and to the transformer for controlling the power supplied to the transformer, temperature sensing means disposed in said zone, means providing a predetermined set point temperature connected to said temperature sensing means for providing a signal proportional tothe difference between the measured temperature and the set point temperature, a galvanometer connected to said temperature sensing means and said means providing a set point temperature, a resistive network and means including a magnetic amplifier connecting said resistive network to said saturable core reactor, said resistive network providing a constant output impedance for said galvanometer and in addition providing means for regulating the signal from said galvanorneter from zero through 100 percent.

2. In a diffusion furnace, means forming at least two zone, heating means disposed in at least two zones in the furnace, a source of power, control means connected to the source of power and to the heating means in one of said zones, additional control means connected to said source of power and to said heating means in another of said zones, temperature sensing means disposed in said one zone, means providing a predetermined reference set point connected to said temperature sensing means for providing a signal proportional to the difference between the temperature measured by the sensing means and the set point, means for supplying said proportional signal to said first named control means to control the application of power to said heating means in said one zone, additional temperature sensing means disposed in said one zone, temperature sensing means disposed in another of said zones and connected to said additional temperature sensing means to provide an output signal which is proportional to the difference between the temperature measured by the additional temperature sensing means and the temperature sensing and the temperature sensing means in another of said zones, and means for supplying said last named proportional signal to said additional control means to control the application of power to the heating means in another of said zones whereby the temperature in another of said zones is slaved to the temperature in said one zone.

3. A diffusion furnace as in claim 2. wherein said means for supplying said proportional signal to said first named control means includes a galvanometer-type controller, an L-pad connected to the output of the controller for regulating the output signal from the controller from zero to 100%, said L-pad presenting a constant output impedance to said controller.

4. A diffusion furnace as in claim 3 together with a precision potentiometer connected to the output of the L-pad for increasing or decreasing said proportional signal to said first named control means.

5. In a diffusion furnace, means forming at least two zones, heating means disposed in at least two of the zones, a source of power, a transformer connected to the heating means in each of said zones, control means connecting the source of power to the transformer associated with one of said zones, additional control means connecting the source of power to the transformer associated with another of said zones, a reference voltage source representing a predetermined set point temperature, temperature sensing means disposed in said one zone and connected to said reference voltage source and providing a signal proportional to the difference between the measured temperature and the set point temperature, means for supplying said proportional signal to said first named control means to control the application of power to said heating means in said one zone, addition-a1 temperature sensing means disposed in said one zone, temperature sensing means disposed in another of said zones and connected to said additional temperature sensing means and providing a signal proportional to the difference between the temperature measured by the additional temperature sensing means and the temperature sensing means in another of said zones, and means for supplying said last named proportional signal to said additional control means to control the application of power to said heating means in another of said zones.

6; A diffusion furnace as in claim 5 wherein said means for supplying said proportional signal to said first named control means and said means for supplying said last named proportional signal to said first named control means each includes a galvanometer-type controller togather with a resistive network connected to the output of 8 the controller, said resistive network providing a constant output impedance to the controller, and means for regulating the output from the controller from zero to 7. A diffusion furnace as in claim 6 wherein said resistive network includes a precision potentiometer for providing fine temperature adjustments.

8. A diffusion furnace as in claim 5 wherein said control means consists of a saturable core reactor and wherein said additional control means also consists of a saturable core reactor.

9. A diffusion furnace as in claim 8 wherein said means for supplying said proportional signal to said first named control means and said means for supplying said last named proportional signal to said last named control means each includes a magnetic amplifier having its output connected to the saturable core reactor and having its input connected to the output of the resistive network.

10. In a diffusion furnace, means forming at least two zones, heating means extending through at least two zones in the furnace, a source of power, a transformer connected to the heating means in each of the zones, control means connecting the source of power to the transformer in one of said zones, additional control means connecting the source of power to the transformer in another of said zones, temperature sensing means disposed in said one zone, means providing a predetermined reference temperature connected to said temperature sensing means and providing a signal in accordance with the deviation between the reference and the temperature sensing means, a controller connected to said temperature sensing means and to said reference for providing a signal proportional to the deviation between the reference and the measured temperature, an L-pad connected to the output of the controller and presenting a constant output impedance to the controller, said L-pad including means for varying the output signal from the controller from zero to 100%, means connecting the output from the L-pad to the control means, additional temperature sensing means disposed in said one zone, temperature sensing means disposed in another of said zones and connected to said additional sensing means to provide a signal in accordance with the deviation between the temperatures measured by the additional temperature sensing means and the temperature sensing means disposed in another of said zones, a controller connected to said additional temperature sensing means and to said temperature sensing means disposed in another of said zones, an L-pad connected to the output of the controller and presenting a constant output impedance to the controller, said L-pad also including means for varying the output from the controller from zero to 100%, and means connecting the output of the L-pad to the additional control means.

11. A diffusion furnace as in claim 10 wherein said means connecting the output of the L-pad to the control means includes a precision potentiometer having one end connected to the L-pad and having its wiper connected to the control means, and wherein the means connecting the output of the L-pad to the additional control means includes a precision potentiometer having one end connected to the L-pad and having its wiper connected to the additional control means.

12. A diffusion furnace as in claim 10 wherein the other end of each of said potentiometer is connected to the wiper of the associated potentiometer.

References Cited by the Examiner UNITED STATES PATENTS 1,391,996 9/1921 Collins 13 24 1,506,443 8/1924 Otis 13-24 1,893,847 1/1933 Simpson 13--24 (Other references on follo wing page) 9 UNITED STATES PATENTS Wunsch 73-341 Wilhjelm 23615 Jung 1324 Anderson 219-497 X 5 Atkins 21950'3 Mitchell 219503 10 2,720,579 10/1955 Morgan 219-503 2,874,906 2/ 1959 Nossen 236-15 3,171,018 2/1965 Lawler 219-494 3,183,294 5/1965 Kasper 1327 RICHARD M. WOOD, Primary Examiner.

V. Y. MAYEWSKY, As istant Examiner. 

1. IN A DIFFUSION FURNACE, MEANS FORMING AT LEAST ONE ZONE, HEATING MEANS DISPOSED IN THE ZONE, A SOURCE OF POWER, A TRANSFORMER CONNECTED TO THE SOURCE OF POWER SATURABLE CORE REACTOR CONNECTED TO THE SOURCE OF POWER AND TO THE TRANSFORMER FOR CONTROLLING THE POWER SUPPLIED TO THE TRANSFORMER, TEMPERATURE SENSING MEANS DISPOSED IN SAID ZONE, MEANS PROVIDING A PREDETERMINED SET POINT TEMPERATURE CONNECTED TO SAID TEMPERATURE SENSING MEANS FOR PROVIDING A SIGNAL PROPORTIONAL TO THE DIFFERENCE BETWEEN THE MASURED TEMPERATURE AND THE SET POINT TEMPERATURE A GALVANOMETER CONNECTED TO SAID TEMPERATURE SENSING MEANS AND SAID MEANS PROVIDING A SET POINT TEMPERATURE, A RESISTIVE NETWORK AND MEANS INCLUDING A MAGNETIC AMPLIFIER CONNECTING SAID RESISTIVE NETWORK TO SAID SATURABLE CORE REACTOR, SAID RESISTIVE NETWORK PROVIDING A CONSTANT OUTPUT IMPEDANCE FOR SAID GALVANOMETER AND IN ADDITION PROVIDING MEANS FOR REGULATING THE SIGNAL FROM SAID GALVANOMETER FROM ZERO THROUGH 100 PERCENT. 